1. Field of the Invention
Aspects of the present invention relate to a laser scanning unit for deflectively scanning incident light, and more particularly, to an improved polygon mirror of a laser scanning unit and an image forming apparatus having the same.
2. Description of the Related Art
In general, a laser scanning unit is employed with a printing apparatus, such as a laser printer, a copier and a fax machine, using electrophotography. The laser scanning unit includes a light source for irradiating laser beams, a polygon mirror for reflecting and scanning the irradiated beams to a predetermined direction, an Fθ lens for converting the reflected beams into parallel beams, and a reflecting mirror for reflecting the beams passing through the Fθ lens to a predetermined destination.
The polygon mirror includes a plurality of reflective surfaces which can reflect incident light. The polygon mirror is rotated at a high speed to reflect the incident light to a predetermined direction.
According to U.S. Pat. No. 5,946,125, a first dielectric layer made of SiO2, a second dielectric layer made of ZnSe (Zinc Selenide), and a third dielectric layer made of SiO2 are sequentially coated on the reflective surfaces of the polygon mirror. In this structure, the aluminum reflective surfaces can be protected. Even if an incident angle of laser beams incident on the reflective surfaces varies from 10° to 50°, S and P polarizations maintain a constant reflectivity due to an interference effect caused by the thickness of the dielectrics.
In the conventional multi-coating type polygon mirror, when the incident angle ranges from 10° to 50°, the S and P polarizations maintain an approximately constant reflectivity. However, about 5% of reflectivity deviation is generated according to wavelengths of the laser beams. This reflectivity deviation restricts the use of the polygon mirror.
Additionally, the conventional multi-coating type polygon mirror maintains approximately constant reflectivity until the wavelength of the laser beam decreases to about 500 nm, but the reflectivity deviation then increases in the 400 nm to 450 nm band. Therefore, the polygon mirror does not function efficiently with a blue laser diode.
Furthermore, ZnSe, which is the material of the second dielectric layer, is expensive and poisonous. It is therefore difficult to handle ZnSe and achieve mass production of the conventional polygon mirror.